Disgraced former CIA Director David Petraeus exchanged a sexually explicit email about having sex under a desk with his mistress and continued to pursue her by bombarding her with thousands of messages even after she had broken off the affair it has been revealed.

The racy communications between the married four-star general and his lover, Paula Broadwell, were uncovered when the FBI began an investigation after they suspected corruption between the pair.

Instead the FBI found evidence of a potentially compromising affair for America's top intelligence official which would have been a breach of national security requirements and led to Petraeus' dramatic resignation yesterday.

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'Affair': Paula Broadwell and David Petraeus, pictured, reportedly had an affair that led to his resignation

Petraeus
stepped down yesterday after confessing to cheating on his wife of 37
years, Holly - behaviour he explained was 'unacceptable' for a senior
administration official.

The
affair was uncovered after the FBI launched an investigation when American intelligence mistook an email Petraeus had sent to his girlfriend as a reference to corruption.

Sources at the FBI told Newsmax that the investigation of the former CIA boss began in spring of this year and that federal agents pored over his emails from that point and from when he was stationed in Afghanistan from July 4th 2010 to July 18th 2011.

Discovering the affair between his biographer and journalist Paula Broadwell, 40, the FBI continued to intercept email between the pair - discovering messages that include sexually explicit references to such items as sex under a desk.

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After being sworn in as CIA Director on September 6th, 2011, Broadwell broke up with Petraeus, but he continued to pursue her, sending her thousands of emails over the last few months - raising questions about his judgement which led in part to his resignation.

Broadwell,
who researched the book 'All In' for three years, had extensive access
to Petraeus in Afghanistan. Yet sources told NBC it is unlikely she will
face criminal charges after the alleged hacking, stressing that
Petraeus himself is under no investigation.

He first met Broadwell six years ago when he addressed students at Harvard University, where she is a researcher, and they eventually got to know each other better during mile-long runs.

Broadwell, who served in the military for more than a decade, lives in Charlotte, North Carolina with her radiologist husband, Dr. Scott Broadwell, and their two young sons, Lucien and Landon.

'Other woman': Paula Broadwell, left, is married and has two children. Petraeus has been married to his wife Holly, pictured right, for 37 years, and they also have two children

Husband and wife: Paula Broadwell pictured with her husband and father of her children, Scott, a radiologist

In the biography, Broadwell revealed she first met him during his visit to Harvard in spring 2006 when he was a Commander at Fort Leavenworth.

'I
was among the students invited by the school to meet with the general
at a dinner afterward, because of my military background,' she
explained.

'I introduced
myself to then–Lieutenant General Petraeus and told him about my
research interests; he gave me his card and offered to put me in touch
with other researchers and service members working on the same issues.'

Broadwell added that this was not a one-off as he often 'does a lot of mentoring'.

As she pursued her PhD in public policy in 2008, she contacted him to ask if she could interview him and they kept in touch via email.

'A few months into my research, General
Petraeus, who was then leading Central Command, invited me to go for a
run with him and his team along the Potomac River during one of his
visits to Washington,' she wrote. 'I figured I could interview him while we ran.'

She explained that, after earning varsity letters in cross-country and indoor and outdoor track, she wanted to test him to see if he could keep up with her as she interviewed him.

Close: Broadwell first met Petraeus when he visited Harvard University, where she was a student

Embedded: Broadwell, pictured in Afghanistan, has more than a decade of U.S. military service

'Instead it became a test for me,'
she said. 'As we talked during the run from the Pentagon to the
Washington Monument and back, Petraeus progressively increased the pace
until the talk turned to heavy breathing and we reached a
six-minute-per-mile pace. It was a signature Petraeus move.'

Promoting the book on The Daily Show
with John Stewart in January, she insisted that these mile-long runs together were nothing out of the ordinary.

'This is a typical mechanism for him
to get to know young people,' she said. 'He's done it throughout his
life. That was the foundation of our relationship.'

In another often awkward radio interview in
January, host Don Imus said that the pair must have 'obviously' liked each other.

She responded: 'You know, as I said
earlier, he has a number of mentees and that's one thing that's
different when you compare him to other Senior Commanders.

Author: Broadwell completed a biography about Petraeus, published earlier this year

'But, yeah, we had a lot of rapport. I
think some of that comes from a common ground of having gone to West
Point [to the U.S. Military Academy].'

Of her reported rival, she added: 'He
is married to Holly Petraeus, who is a wonderful Military spouse and
done so much for their children and for children of fallen soldiers, I
respect her immensely.'

The book's description explains Broadwell conducted research for three years and was 'afforded extensive access by General Petraeus, his mentors, his subordinates and his longtime friends'. Its website adds Broadwell was embedded in Afghanistan with Petraeus for a year between July 2010 and 2011.

Broadwell graduated with honors from the U.S. Military Academy in West Point, and came top of her class in physical fitness. Petraeus also attended the academy and married his wife Holly there.

She has more than a decade of military service and nearly two decades of work in counterterrorism and counterinsurgency.

She is studying for her PhD at Kings College in London, and is also a research associate at Harvard University's Center for Public Leadership. For her studies and the biography, she spent most of 2011 in Afghanistan as an embedded author.

As well as contributing to book chapters, appearing on television interviews and writing opinion pieces for The New York Times and the Boston Globe about leadership and women in defense, she has also worked as a model and demonstrator for KRISS, a .45 caliber machine gun manufacturer.

Last week, Broadwell wrote an article entitled 'David Petraeus's Rules for Living' in Newsweek.

Indeed, one military source who has worked with Petraeus said that Broadwell's attempts to 'get her claws into' the CIA chief were an open secret.

'When she started work on the bio she called me for background on one of the general's previous deployments.

'I probably gave her four hours or so, and we stayed in touch after that by email and an occasional phone call.

'Over that time, she went from someone very likeable to a shameless self-promoting prom queen.

'A very disturbing shift in how she carried herself. If she knew P4 was going to make an appearance at an event, she'd crash it without an invitation (she actually did this at the wedding of some close friends of mine) and photo bomb[ed] sic everyone there.

Ally: Barack Obama could be harmed by the loss of one of his senior national security officials

One read: 'We all will make mistakes. The key is to recognize them and admit them, to learn from them, and to take off the rear view mirrors - drive on and avoid making them again.'

Petraeus had been sworn in as head of the CIA in September 2011 after serving as
head of the coalition forces first in Iraq and then in Afghanistan.

President Obama accepted Petraeus' resignation but described him as 'one of the outstanding General officers of his generation' as he added: 'By any measure, through his lifetime of service David Petraeus has made our country safer and stronger.'

Deputy director Michael Morell will serve as acting head of the agency until a permanent replacement for Petraeus is appointed.

In his resignation letter addressed to CIA agents, Petraeus wrote that 'such behaviour is unacceptable' for a senior administration official.

The fact of an affair would not in
itself normally merit a resignation, unless the affair created a
security risk, for instance if it was with a foreigner or a journalist.
Blackmail would also be an issue, though publicly admitting the affair
would prevent that.

Sweethearts: Petraeus on the day he graduated from the U.S. Military Academy in 1974, pictured with his fiancée Holly Knowlton, whose father was the superintendent of West Point

Oath of office: Petraeus was sworn in last September by Joe Biden with his wife Holly at his side

An affair with a subordinate CIA employee might make a resignation almost inevitable - or it is possible that Petraeus stepped down because he felt his leadership and integrity had been compromised.

One of the most puzzling aspects of
the resignation is its proximity to next week’s congressional hearings
on Benghazi, which Petraeus has said he will not now attend, even though
being out of office would not preclude his doing so.

LIFE IMITATES ART: HOW CIA AFFAIR SCANDAL IS ECHOED IN HOMELAND

David Petraeus' confession that he has had an affair mirrors the plot of Showtime's Emmy-winning drama Homeland.

David Estes (above, played by David Harewood), CIA director of counter-terrorism in the show, has an extra-marital affair with lead character Carrie Mathison (Claire Danes).

Although the fling leads to the break-up of his marriage, there is no suggestion that either he or Carrie should their job over it.

Petraeus
admitted he had shown 'extremely poor judgement' in the letter he sent
to his CIA colleagues confirming that he was stepping down.

He added that it had been 'the greatest of
privileges' to work at the agency, saying: 'Teddy
Roosevelt once observed that life's greatest gift is the opportunity to
work hard at work worth doing.'

Petraeus,
60, has been married to Holly, née Knowlton, since 1975, after they
meet when he was a military cadet at West Point and she was the daughter
of the academy's superintendent.

When
he was sworn in as director of the CIA by Vice President Joe Biden,
Holly held the Bible on which he swore his oath of office.

Mrs Petraeus is head of the Office of Servicemember Affairs, a department of the Consumer Financial Protection Bureau which is responsible for ensuring members of the military are not taken advantage of by banks and other financial businesses.

The official offered his resignation to Obama on Thursday, and the President accepted the offer on Friday.

Obama issued a statement thanking Petraeus for his 'extraordinary service', saying he had 'served with characteristic intellectual rigor, dedication, and patriotism'.

The President added: 'Going forward, my thoughts and prayers are with Dave and Holly
Petraeus, who has done so much to help military families through her
own work. I wish them the very best at this difficult time.'

Confident: Paula Broadwell appeared on The Daily Show with John Stewart in January to promote her biography of David Petraeus 'All In'

Comfortable: She lives with her husband and their two young sons in Charlotte, North Carolina, pictured

Director of National Intelligence James Clapper had even stronger praise for Petraeus, describing him as 'one of our nation's great patriots' as he added, 'Dave has redefined what it means to serve and sacrifice for one's country.'

Clapper continued: 'Whether he was in uniform leading our nation's troops in Iraq and
Afghanistan, or at CIA headquarters leading the effort to generate
intelligence used to keep our nation safe, Dave inspired people who had
the privilege of working with him.'

Dianne Feinstein, a Democratic senator from California who is chair of the Senate Intelligence Committee, said: 'I wish President Obama had not accepted this resignation, but I understand and respect the decision.'

She added: 'I wanted him to continue. He was good, he loved the work and he had a command of intelligence issues second to none.'

Peter King, the Republican who chairs the House Committee on Homeland Security, said: 'General Petraeus is one of America’s most outstanding and distinguished military leaders and a true American patriot.'

'I SHOWED EXTREMELY POOR JUDGEMENT': RESIGNATION LETTER

Yesterday
afternoon, I went to the White House and asked the President to be
allowed, for personal reasons, to resign from my position as D/CIA.
After being married for over 37 years, I showed extremely poor judgment
by engaging in an extramarital affair. Such behavior is unacceptable,
both as a husband and as the leader of an organization such as ours.
This afternoon, the President graciously accepted my resignation.

As
I depart Langley, I want you to know that it has been the greatest of
privileges to have served with you, the officers of our Nation's Silent
Service, a work force that is truly exceptional in every regard. Indeed,
you did extraordinary work on a host of critical missions during my
time as director, and I am deeply grateful to you for that.

Teddy
Roosevelt once observed that life's greatest gift is the opportunity to
work hard at work worth doing. I will always treasure my opportunity to
have done that with you and I will always regret the circumstances that
brought that work with you to an end.

Thank
you for your extraordinary service to our country, and best wishes for
continued success in the important endeavors that lie ahead for our
country and our Agency.

David Petraeus has provided
extraordinary service to the United States for decades.

By any measure,
he was one of the outstanding General officers of his generation,
helping our military adapt to new challenges, and leading our men and
women in uniform through a remarkable period of service in Iraq and
Afghanistan, where he helped our nation put those wars on a path to a
responsible end.

As
Director of the Central Intelligence Agency, he has continued to serve
with characteristic intellectual rigor, dedication, and patriotism.

By
any measure, through his lifetime of service David Petraeus has made our
country safer and stronger.

Today,
I accepted his resignation as Director of the Central Intelligence
Agency.

I am completely confident that the CIA will continue to thrive
and carry out its essential mission, and I have the utmost confidence in
Acting Director Michael Morell and the men and women of the CIA who
work every day to keep our nation safe.

Going
forward, my thoughts and prayers are with Dave and Holly Petraeus, who
has done so much to help military families through her own work. I wish
them the very best at this difficult time.

Man of action: Petraeus guided Obama around Iraq when he visited as a presidential candidate in 2008

Appointment: The general served under George W. Bush as commander of U.S. forces in Iraq

The resignation comes at an awkward
time for the President, who is beginning to put together his team for
his second term in the wake of his re-election on Tuesday.

Secretary
of State Hillary Clinton has already declared her plans to step down in
January, while Secretary of Defense Leon Panetta is expected to follow
some time in 2013.

Petraeus' departure leaves another vacancy for a key position in the national security establishment.

Morell,
who has served as the agency's deputy director for two and a half
years, will be one of the frontrunners to take over on a permanent basis
- former CIA director George Tenet was promoted from deputy in the same
way after the resignation of John Deutch in 1996.

He
is a long-serving analyst who has twice been a witness to history,
travelling with George W. Bush on the day of the September 11 attacks
and sitting in the White House situation room with Barack Obama during
the mission to kill Osama Bin Laden in May last year.

Petraeus
was in charge of the CIA at the time of the terror attack on the U.S.
consulate in Benghazi on September 11 of this year, a raid in which
ambassador Chris Stevens and three other Americans were killed and which
has become a political disaster for the Obama administration.

The
former CIA director was due to address a congressional hearing on the
attack next week, which could raise suspicions that the timing of his
resignation was politically motivated.

He will no longer be present at the hearing, which will instead grill Morell on the agency's failings in allowing four Americans to die during the assault on the consulate and a CIA compound.

Despite
his close working relationship with Obama and other Democratic
officials, Petraeus was spoken of as a possible running mate for
Republican presidential candidate Mitt Romney this year.

THE TOP SOLDIER OF HIS GENERATION: PETRAEUS' STARRY CAREER

Hero: David Petraeus has an unparalleled reputation among soldiers of his generation

David Howell Petraeus is a retired U.S. Army four-star general and the most celebrated American soldier of his time.

Known
as a scholar and a warrior, the West Point graduate is admired as much
for his intellect as his tactical ability and charisma on the
battlefield.

He oversaw
the crafting of the successful 2006 U.S. counterinsurgency measures in
Iraq and accepted without hesitation Obama's request to leave Central
Command in 2010 and lend his expertise to the failing international
effort in Afghanistan.

Spending
37 years in the military, Petraeus retired from service and took up his
post as director of the CIA on September 6, 2011.

Born
in Cornwall-on-Hudson in New York in 1952, Petraeus is the son of small
town librarian Miriam and Sixtus, a sea captain who had emigrated from
the Netherlands.

Immediately
upon his graduation from high school in 1970 he enrolled as a cadet at
West Point Military Academy where he served with distinction -
graduating in the top five of his class in 1974.

Just after his graduation he married Holly Knowlton, daughter of West Point superintendent General William A. Knowlton.

Leaving West Point, Petraeus embarked on his military career and was commissioned as a second lieutenant with the Army Rangers.

Command: Petraeus took over as head of coalition troops in Afghanistan in 2010

He served in a light infantry airborne battalion combat team in Italy and by 1978 he had attained the rank of captain.

His rise though the ranks was rapid. By 2000 he was a brigadier general, and in January 2007 he became a four-star general.

Continuing
his education at the same time as his active service, Petraeus was
recognised for his intellect and won the General George C. Marshall
Award as the top graduate of the U.S. Army Command and General Staff
College in 1983.

This
allowed him to study at the university of his choice and he enrolled at
the School of International Affairs at Princeton University - earning a
Master's degree in 1985 and a PhD in 1987.

Beginning his command career in Haiti in 1995 as NATO's chief operations officer, Petraeus served in Kuwait and Bosnia.

Spouses: David and Holly Petraeus have been married for over 37 years

In 2007 he commanded the Bush administration's successful deployment to Iraq of 20,000 new troops to combat rising insurgents.

His counterinsurgency strategy has become known as the 'Petraeus Doctrine'.

By 2008, Petraeus was the
United States' top soldier and took the reins of the U.S. based Central
Command, as USCENTCOM, in October of that year.

However,
following Afghanistan commander Stanley McChrystal's negative comments
regarding the Obama administration in 2010, Petraeus was asked to take
control of the international force there.

He
continued to serve in this capacity until he left the military on
August 31, 2011 - where at his retirement, Admiral Michael Mullen,
Chairman of the Joint Chiefs of Staff compared him to legendary Civil
War general and U.S. President Ulysses S. Grant.